The sewage disposal apparatus employing large filter media has a filter bed formed by an assembly of a multiplicity of buoyant filter media in a septic tank, with aerobic or anaerobic bacteria which are effective for sewage treatment being allowed to proliferate on this filter bed, and sewage is designed to be purified gradually as it is allowed to flow slowly downstream in the septic tank in contact with the filter media in the filter bed.
However, in the prior art sewage disposal apparatus employing large filter media, the filter bed is fixed, so that sludge clogging occurs in each medium or between the media to reduce purifying capacity of the apparatus after a predetermined amount of treatment. In order to eliminate such reduction in the purifying capacity, the filter media in the apparatus must entirely be replaced with new ones or cleaned and the apparatus must be overhauled periodically, leading to great operation cost elevation.
There are other conventional methods which prevent such clogging by employing filter media hard to be clogged or by pretreating sewage to reduce the sludge concentration thereof, followed by treatment of the diluted sewage for final sewage disposal treatment.
However, the conventional method employing pretreatment of sewage requires considerably troublesome work for the pretreatment itself, and all what it can do is to delay clogging. Thus, it is impossible to thoroughly obviate clogging itself, and eventually, the filter media must be replaced with new ones or cleaned periodically, which requires troublesome, heavy work.
Meanwhile, a buoyant, spherical filter medium hard to agglomerate itself with others is disclosed, for example, in Japanese Patent No. 1343032 (Japanese Patent Publication (Kokoku) No. 61-2440). Such contrivance may impart a high level of clogging inhibitory capacity to the medium itself, but it is not sufficient to completely eliminate clogging.
The present invention has been made in view of such problems in the prior art, and it is therefore an object of the invention to provide a sewage disposal apparatus, in which a gentle flow of the filter bed is provided by a circulatory movement of the filter media, while the sewage under purification treatment flowing through the filter bed is adapted not to be affected, or the flakes of sludge coming off from the filter media are prevented from diffusing into the treated water as the filter media are moved, and further the filter media are cleaned by utilizing this circulatory movement thereof whereby to prevent effectively the clogging of the filter medium as well as to constantly enable efficient sewage disposal treatment.